1. Field of the Invention
The present invention relates to an apparatus for displaying a video image and, more particularly, to a head-mounted image display apparatus (head-mounted display), a hand-held image display apparatus (hand-held display), and an image projecting apparatus.
2. Description of the Related Art
There have conventionally been proposed head-mounted image display apparatuses (head-mounted displays) and hand-held image display apparatuses (hand-held displays) which guide image light to the pupils of an observer and displays a virtual image. Japanese Patent Laid-Open No. 11-95144 proposes an image display apparatus for displaying an image based on a light beam reflected by a reflection surface that makes a reciprocating rotation.
The image display apparatus described in Japanese Patent Laid-Open No. 11-95144 includes one horizontal scanning unit (shared by left and right eyes) and two vertical scanning units. The horizontal scanning unit and the vertical scanning units scan, via a collimator lens, light beams from two left- and right-eye light sources. An optical system then guides the light to a user's eyes.
An image display apparatus described in Japanese Patent Laid-Open No. 2006-162780 uses a two-dimensional scan mirror, with the goal of optically reducing the apparatus size.
However, neither patent reference includes a detailed description of light source control. Japanese Patent Laid-Open No. 2004-517350 describes an image display apparatus which scans a plurality of light beams simultaneously and arranges images in tiling display.
Japanese Patent Laid-Open No. 2006-220745 presents a problem in a micromirror scanner using the resonance phenomenon of a silicon substrate. Young's modulus of a silicon substrate changes as temperature changes, and the resonance characteristic (maximum resonance frequency) also changes accordingly. At a constant oscillation frequency (when an applied AC voltage has a predetermined frequency), the oscillation angle of a micromirror varies as the temperature changes. For example, even when the temperature of the micromirror changes, the oscillation frequency does not change as long as the frequency of the applied AC voltage is constant. However, as the temperature of the micromirror rises, the oscillation angle of the micromirror decreases. The variations in the oscillation angle of the micromirror cause variations in the scan range of the reflected light of a laser beam emitted toward the micromirror. If there are periods when both the left- and right-eye laser beams irradiate the micromirror and periods when neither the left- nor right-eye laser beam irradiates it, the temperature of the micromirror scanner varies over time. The temperature unevenness appears as image distortion.
In the reciprocating scanning application method for a tiling display described in PCT(WO) 2004-517350, the scanning slope of the left video image and that of the right video image are different. The scan positions in the overlap portion of the left and right video images also shift almost by half of a line. This causes a scan shift at the overlap portion of the left and right video images and consequently makes the joint noticeable. Additionally, since scanning of the right video image is done in a direction opposite to the normal raster scanning direction, each light source requires a buffer.